Inadequacy of 1D, 2D and 3D Resistivity Inverse Modelling in the Presence of Electrical Anisotropy

Branislav Sretenovic, University of Belgrade, Faculty of Mining and Geology, Department of Geophysics, Belgrade, Serbia
Filip Arnaut, University of Belgrade, Faculty of Mining and Geology, Department of Geophysics, Belgrade, Serbia

Universal property of matter is the variation of a certain physical characteristic in different direction. The Substances that do not display this property are an exception. Anisotropy, as this property is named, is also notable in electrical conductivity of minerals, ores, rocks and geological formations. In order to properly define a geological model of an investigated area, it is necessary to account for electrical anisotropy and lateral effects of different origin that are almost always present phenomena. The degree of knowledge of these phenomena determines the quality of interpretation. The effects of electrical anisotropy on 1D, 2D and 3D inversion of apparent resistivity data were examined and the way to detect, quantify and analyse electrical anisotropy in 3D case is proposed. The results of this analysis showed that electrically anisotropic models have led to the totally erroneous results of 3D inversion while the effects of electrical anisotropy on 1D and 2D inversion were less pronounced. Among several existing ways of collecting 3D apparent resistivity data the complete Pole-pole array data set is the only one suitable for detection of electrical anisotropy. Pole-pole resistivity data enable calculating of corresponding resistance or apparent resistivity values for all other collinear or square arrays. This fact makes possible to use standard grid of electrodes in performing 3D Pole-pole apparent resistivity measurements for calculating square array apparent resistivity data. In the special case of vertical stratification (fracturing, schistosity..) this calculated square array data were used for electrical anisotropy analysis thus determining values of coefficient of anisotropy () and mean geometric resistivity (m). The simple two and three layer 1D synthetic anisotropic models, were used to determine parameters of electrical anisotropy by using 3D forward modelling to calculate Pole-pole and then square array apparent resistivity values. The mean geometric resistivity data obtained by using square array, which are orientation-independent, were used for 1D inversion leading to more realistic results. In the case of oblique stratification (lamination, fracturing, schistosity or karstification) crossed square resistivity data can be reconstructed from 3D Pole-pole data set and then can be used to get parameters of electrical anisotropy, namely apparent coefficient of anisotropy (n), mean geometric resistivity (m) and apparent electrical strike (). These parameters provide an indication that investigations of electrical anisotropy should be conducted (by using square and crossed square array) in order to avoid erroneous results of 3D inversion.